1. Field of the Invention
The present invention relates generally to a gear transmission for transmitting power from a power source to an output shaft; and more specifically, to an epicyclic gear transmission having tilted axis planets.
2. Description of Related Art
Modern rotary wing aircraft; i.e., helicopters, typically utilize a gas turbine engine to drive a rotor. A gearbox located between the engine and the rotor transfers power and functions as a speed reduction unit. The gearbox or main rotor gear transmission is normally the heaviest single subsystem in the drive system of the aircraft.
Currently, the final transmission stage in virtually all main rotor drives is an epicyclic transmission system that typically consists of one or two simple, spur gear planetary stages. These planetary gear stages are composed of a sun gear driven by the input shaft of the transmission, multiple planet gears (typically between three and six) that intermesh with the sun gear and are spatially arranged around the periphery of the sun gear, and an orbit gear or internal ring gear that intermeshes with and surrounds the multiple planet gears. The orbit or internal ring gear is generally the fixed member of the epicyclic transmission system. The multiple planet gears are mounted to a carrier that in turn is operatively connected to the output shaft of the transmission. While this basic epicyclic gear transmission provides relatively good power efficiency, it has a tendency to generate high noise levels in operation due to the limited contact ratio of the sun gear and the internal ring gear with the multiple planet gears.
U.S. Pat. No. 6,966,865, the disclosure of which is hereby incorporated by reference in the present application, discloses one type of a gear transmission for use with a rotary wing aircraft. The '865 patent illustrates a gear transmission including a compound epicyclic gear system using a planet gear shaft having a larger planet gear and two smaller planet gears on the same planet gear shaft. The two smaller planet gears may be configured as double helical planet gears that are axially staggered in their positions on the planet gear shaft. The epicyclic gear transmission of the '865 patent has the basic components of an epicyclic gear system including a sun gear input, a fixed ring gear or internal gear, and a planet gear carrier output. As disclosed, the planet gear of the epicyclic gear system is a planet gear cluster that includes the smaller double helical planet gears and the larger planet gear both located on the same planet gear shaft. The smaller double helical planet gears are axially spaced from the larger planet gear. The sun gear meshes with the larger planet gear and the fixed ring gear meshes with the smaller planet gears. The planet gears have 2 distinct variations, there is a long planet and a short planet which allow the planets to be intermeshed. Intermeshing of the large gear end of the planets results in smaller packaging of the transmission.
While such an epicyclic gear transmission is suitable for use in a rotary wing aircraft, a reduction in package or envelope size and corresponding weight reduction is desirable. Accordingly, design improvements that reduce the footprint of the gear transmission along with the weight thereof provide an additional advantage for current epicyclic gear transmissions.